Arrangement for concentrating sea waves

ABSTRACT

An arrangement for concentrating sea waves, includes a grid-like structure of stopping and/or delaying elements adapted to influence the amplitude and/or phase of the waves and located in such positions in the water that the elements in interaction with the incident sea waves form an interference pattern which gives a concentration of the wave energy in a limited area (concentration area). When the wave energy is to be utilized for power production, the grid-like element structure is designed to give a concentration of the wave energy in a concentration area located in the short-range field of the element structure. Preferably the elements in the structure are mainly situated in front of the concentration area with respect to the dominant wave incident directions. Moreover, the element structure may have an extension (aperture) which is substantially larger than the wave length of the dominant waves at the location.

BACKGROUND OF THE INVENTION

In the past a number of proposals have been set forth for theutilization of the energy in water waves, in particular sea waves.Energy production on the basis of sea waves has attracted renewedinterest during recent years because of future energy supply problems.

A disadvantage of sea energy as it is available in nature is that theenergy density is low, which is reflected in the rather extensive andthereby expensive constructions which have been proposed fortransforming the energy of the wave motion to electric power forexample. Certain attempts to obtain a collection or concentration of thewave energy over a length of the wave front are shown for example inU.S. Pat. No. 2,441,759 and German published patent application No.2,507,330. Both these proposals are directed to the employment offunnel-shaped channels which narrow towards a restricted area in whichthe utilization of the wave energy can take place. It is evident thatconstruction of this kind cannot in practice be built with dimensions ofa significant size for power supply purposes, since technicaldifficulties and economic considerations will set a limit to sucharrangements.

SUMMARY OF THE INVENTION

The present invention is based on a quite different idea than the aboveprevious proposals and opens possibilities for collecting orconcentrating sea wave energy from considerably larger lengths of thewave front without increasing the construction expenses correspondingly.Moreover, this invention can also be used for establishing quiet, moreor less wave-free areas in the sea, for example at harbours and quaiesor at oil drilling platforms and the like. What is contemplated here isan effect which is complementary to the above mentioned concentrationeffect and can apparently be utilized in combination therewith, i.e. byconcentrating the energy at a wave power plant and to utilize theresulting wave-free area for harbour purposes and the like. Anotherpossible utilization is for ice breaking, for example by concentratingthe energy to a channel-shaped area bounded by two substantiallyparallel lines so that a corresponding open lane is broken through theice.

What is novel and specific in an arrangement for concentrating sea wavesaccoridng to the present invention primarily consists in a grid-likestructure of .[.stopping and/or.]. delaying elements adapted toinfluence the .[.amplitude and/or.]. phase of the waves and located insuch positions in the water that the elements in interaction with theincident sea waves form an interference pattern which gives aconcentration of the wave energy in a limited area (concentration area).

In arrangements where the wave energy is to be used for power productionit is an important feature according to the invention that the elementstructure is adapted to give a concentration of the wave energy in aconcentration area located in the short-range field of the elementstructure. In .[.order.]. .Iadd.other .Iaddend.applications, for exampleice breaking, also the distant field of the interference pattern fromthe element structure may be utilized, as the strong concentration whichis only possible in the short-range field, in many instances is notnecessary in order to obtain a sufficient effect of the kind desired.

Accordingly, this invention is based on the employment of a grid-likestructure or diffraction structure in analogy with structures known fromoptics or from antennas for electromagnetic waves for communicationpurposes. By suitable form and dimensions of such a diffractionstructure for water waves it is possible to establish interferencepatterns, inter alia, for concentrating the wave energy in a small area.Such a grid structure does not necessarily have to take a simplegeometric shape, for example with elements being located along astraight or curved line, but can also be brought to give a desired formof wave .[.inerference.]. .Iadd.interference .Iaddend.in water byarranging the elements in the structure in various and different ways.

When there is here referred to "stopping or delaying elements" it is tobe understood that the elements may be adapted to influence theamplitude and/or phase of the waves. Thus, a stopping element may serveto influence the wave amplitude to such a degree that the waves arelocally completely stopped, i.e. that the amplitude behind the elementis reduced to zero, whereas the waves are passing through at otherplaces in the grid and contribute to the establishment of an inerferencepattern. Delaying elements are elements which influence the phase of thewaves and thereby contribute to the establishment of an interferencepattern. In this connection it is very important to note that elementsof the type which form a phase grid will have a much higher efficiencyin such arrangements than stopping elements which only influence theamplitude of the waves. Therefore, there is every reason to prefer oraim at an element structure which mainly influences the phase of thewaves. In practice the elements employed may have influence both onamplitude and phase, but it is preferred to a largest possible degree tolet such arrangement be based on influencing the phase of the waves.

The reason why a phase grid or delaying elements are more efficient isthat they do not significantly reduce the flow of wave energy throughthe structure. Stopping elements, however, will always reflect some waveenergy so that less energy will flow through the structure.

A single installation, structure or arrangement may include stoppingelements (influencing amplitude) as well as delaying elements(influencing phase).

It is the short-range field of the wave interference pattern which is ofinterest in connection with the concentration of sea waves for energyproduction, as an essential idea of this invention consists therein thatin analogy with optical microscope geometry it is possible to arrangethe element structure in such a way that waves with various incidentwave directions are focused at substantially the same point orconcentration area, which can also be referred to as a focal area. Thisfocal area therefore is that area in the short-range field of theelement structure which is covered by the various and adjacent focalpoints of differently incident waves against the element structure. Asfar as the distant field is concerned the various focal points inpractice will have so large a spacing that a sufficient concentrationfor power production may not be obtained. Besides, the distant fieldwill usually be so much disturbed by extraneous incident waves that theintended concentration in a limited area is still more reduced.

What is explained immediately above will be evident to any one familiarwith wave theory by analogy with optics or other electromagnetic waves.It shall be pointed out, however, that utilization of the short-rangefield is not absolutely necessary for wave energy production, but it ismuch preferred to locate the focal area in the short-range field inarrangements for power production purposes.

The most important factor or parameter affecting the short-range fieldis the element structure's ability to concentrate waves with variousincident wave directions in the concentration of focal area mentionedabove. Another parameter of importance is that within the limitationsdefined below waves of different frequencies will concentrate to asatisfactory degree.

When a given installation or arrangement is being planned, computationswill be carried out in a way similar to what is traditional in thebuilding or mechanical arts, i.e. by choosing a configuration anddimensions and then calculate the consequences thereof, and inconnection with the present invention one such consequence or result isobtained by means of the rule referred to below. If the configurationand dimensions (parameters) first chosen do not satisfy therequirements, inter alia, what is required by the rule below, anotherchoice must be made and the computations repeated until a satisfactoryresult is obtained.

How far the short-range field extends in relation to the diffractionstructure in water may vary. In connection with this invention it hasbeen found, however, that for the plurality of practical cases it willbe correct to set a limit to the extension of the short-range field bythe following rule:

If the largest dimension across the whole element structure is D_(max)and the distance between the concentration area and the closest elementin the structure is A_(min), then the concentration area is inside theshort-range field when ##EQU1##

The above "largest dimension" is the distance between the two elementsbeing most remote from each other in the structure, which usually willbe the length of the structure in a direction normal to the dominantwave propagation direction. How far the concentration area shall bepositioned from the element structure within the above limitations ofthe short-range field depends upon many factors, such as bottomtopography and wave characteristics at the location of the installation.

It is preferred that at least some of the elements are adjustable, forexample by being displaceable as to position and orientation in space,for adaption and adjustment for example in order to take into accountchanges in the direction of the incident waves, but in particular totake into account large of destructive waves which may occur in badweather. By concentration of such large waves the destructive effect maybe catastrophic and it is therefore in such situations desirable to beable to modify or cancel the concentrating effect of the structure.

The construction of the elements may be varied in several ways. Theelements may be mounted to stand fixed on the sea bottom, or they may beanchored, possibly held in position by means of dynamic positioningsystems. Thus, for example, according to the circumstances the elementsmay be formed by concrete blocks or regular vessels, i.e. large shipssuch as tankers or the like (alternative as wrecks) which are suitablypositioned so as to form a grid structure of the desired configuration.As another extreme it is mentioned that also naturally existingelements, such as islets and skerries or the like may be utilized aselements in the grid structure at the actual place for the arrangement.In such cases the positioning of the artificial elements must bedetermined in relation to the already existing natural elements. Thesenatural elements can of course according to the circumstances and theexisting possibilities, be modified by blasting or adding structure sothat a suitable effect thereof is obtained. Although elements arementioned here which individually act separately in the grid structurein co-operation with other elements, there may be .[.though.]..Iadd.thought .Iaddend.of element constructions which form a continuousstructure, for example an elongate hose-like structure which at certainpoints or portions protrudes towards or above the water surface to formseparate elements, but which for the rest are so much immersed that theeffect of the construction on the waves in the intermediate portions isnot noticeable.

As will have appeared from the above there are many degrees of freedomas regards the construction and the arrangement of the elements in agrid structure. What is contemplated here is in the first placeposition, not only in the horizontal plane, but also vertically, i.e.with respect to the water surface, and in the second place orientation,i.e. in azimuth and/or elevation in so far as one does not have to dowith elements of a spherical symmetry, and finally the elements may havethe most different shape and sizes depending upon the effect desired inthe grid structure. In addition to the availability of theseconstruction parameters or variation possibilities for fixed andstationary elements, there may be built elements which are adjustable inthe above respect, i.e. as to position, orientation, shape and size.Thus, an adjustable shape and size can be thought of in connection withthe above hose-like structure.

When employing the here proposed separate elements in a grid-likestructure it will be understood that arrangements for energy productionon the basis of sea waves may have a very large length or extension, forexample of the order of magnitude of several kilometers or possibly asmuch as several tens of kilometers, in particular when naturallyexisting elements may be utilized, such as coral-reefs with openingsblasted out. It is therefore by means of the invention possible in asingle power plant to utilize the wave energy across a far larger regionor coast length than what has been considered possible technically andeconomically by means of previously proposed solutions.

Even though installations for energy transformation as such are notincluded by the present invention, some possibilities are brieflymentioned here. The element structure can for example be arranged insuch a way outwardly of a coast line that the concentration or focalarea is positioned in an inlet or a bay which ends in a natural orblast-out cleft which leads to a higher basin the head of which can beutilized in a turbine installation of conventional type. By arrangementat sea far from the shore the concentrated wave energy may be utilizedby means of a construction similar to a drilling platform having a largehydraulic pump or scoops between its legs, possibly with water liftingequipment and with a basin mounted thereon, for the production ofelectric power. The latter can be transferred to the shore through a seacable.

When using the present invention for power production it is veryimportant to be able to adjust the element structure for each individualplace or site, or that the diffraction structure may be optimized forobtaining the best possible concentration of the energy in a limitedarea in which the transformation to useful power shall take place. Thereis a large number of factors which must be taken in account in theplanning of such an arrangement and mathematical calculations play animportant role for the determination of the configuration of thestructure and the form of the elements. The fundamental mathematical andtheoretical apparatus for the treatment of wave fields of the kindconcerned here has in principle been available for a long time, and hasbeen treated for example in G. B. Whitham: Linear and Nonlinear Waves(Chapter 13), London 1973. Reference is made in particular to thedifferential equations 13.22 on page 437 of this book. Thesedifferential equations must be solved taking into account the boundaryconditions arising from the desired configurations of the elementstructure.

The previously mentioned computations or calculations are made on thebasis of the above equations and the parameters involved are shape, sizeand position of the elements (whether artificial or natural formations)to be utilized. As to the waves the single significant parameter istheir phase. What is essential is to establish an element structurehaving such influence on the incident waves that the resulting outputwave has a circular wave front, which leads to concentration of the waveenergy in the focal area.

It is important during the calculations on the basis of the theoreticalfundament to consider closely which approximations are acceptable inorder that the result shall be useful in practice for the purposesconcerned here. It is, moreover, of substantial significance for thisinvention to recognize certain physical conditions in sea waves, namelythe frequency spectrum thereof and the angular spread of directionsaround the main direction within which the waves usually fall at a givenplace. When in this connection reference is made to sea waves, thismeans the long and powerful swells which more or less at all times rollacross the larger ocean regions and eventually fall against some coast.After systematic measurements and analyses it has been found that suchwaves have a rather narrow band frequency spectrum, which has animportant bearing on the usefulness of this invention, as a higherefficiency is obtained with a narrower frequency spectrum.

A realistic, practical example in this regard may be as follows. Interms of period, which is the inverse of frequency, a common wave periodis 10 sec. An element structure according to this invention will thenresult in an acceptable concentration of wave energy with a deviation inperiod of 20 percent, i.e. for a range of periods between 8 sec. and 12sec. As to angular or directional deviation, a range of angles up to 30°to both sides of the center line or main direction is acceptable. Thefigures given as examples here correspond with the limit of about 20°given above with respect to the short-range field. It will be understoodthat the design of the element structure is less complicated if theangular spread or deviation of wave directions from the main directionis small. Waves that are incident from directions outside the angularrange for which the structure is designed, will not be focused properly.

When energy is to be extracted from the waves in the short-range fieldthe means or equipment provided for this purpose will also have aninfluence on the wave interference pattern. The method of extractingenergy must therefore be taken into account when designing the elementstructure. This is in analogy with what is known in connection withelectromagnetic waves.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention shall now be explained by means of some exemplaryembodiments thereof, with reference to the attached drawings, in which:

FIG. 1 is a schematic illustration of a known diffraction structure fordiffracting light waves,

FIG. 2 is a schematic illustration of a known diffraction structure forfocusing light waves,

FIG. 3 is a schematic illustration of the principle for focusing waterwaves,

FIG. 4 is a perspective view of a set-up used during experiments fortesting an amplitude grid,

FIG. 5 is a perspective view illustrating sea wave concentration inpractice by means of an amplitude grid,

FIG. 6 is a perspective view showing the employment of a phase grid,

FIG. 7 is a schematic view illustrating the utilization of naturallyexisting stopping elements,

FIG. 8 is a perspective view of the utilization of a modified coral reefas an amplitude grid,

FIG. 9 is a schematic view showing alternative delaying elements,

FIG. 10 is a schematic view showing alternative stopping elements,

FIG. 11a and 11b are schematic views showing a combined arrangement, and

FIG. 12 is a graph showing in detail the contour of delaying elementscalculated for a practical arrangement.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows the principle for a usual optical grating or grid 11 inwhich the distance between the points of the grating is constant. Planelight waves 12 which hit the grating will be diffracted in certaindirections as indicated by arrows 13 and 14, determined by the wavelength of the plane waves 12 and by the distance between the points ofthe grating. It is known that a corresponding grating or grid in waterwill deflect water waves in a corresponding way.

In FIG. 2 there is shown a diffracting structure 21 in which the gridpoint spacing decreases from the center and outwardly similar to thestructure of a Fresnel-zone plate. With a correctly determined structureplane light waves 22 which hit the grid 21 will result in convergingspherical waves 23 and a portion of the energy will be focused in anarea 24. The idea behind the present invention is to utilize thisoptical phenomenon for concentrating sea wave energy for powerproduction.

Extensive calculations and experiments have shown that water waves canbe focused by means of a diffraction structure in the same way as lightwaves. The principle of water wave focusing is shown in FIG. 3. Thediffraction structure is also here a grid-like structure 31 whichconsists of stopping or delaying elements 31a adapted to influence theamplitude and/or phase of the water waves. The elements are dimensionedand positioned in such a way in the water that in interaction withincident water waves 32 they form an interference pattern to the effectthat the plane water waves 32 become converging circular waves 33 whichgive a concentration of the wave energy in a limited area in which apower station 34 can be located.

For power production purposes the element structure is most preferablycalculated for concentrating the wave energy in a focal area located inthe short-range field of the element structure. Whether the elementstructure shall exist in the form of stopping elements or delayingelements will to a certain degree depend on the topographical conditionsat the bottom of the sea where the structure shall be located. In otherwords the natural formations may lend themselves to utilization eitheras stopping elements or as delaying elements, the latter beingnecessarily more or less flush with the water, such as elements 95 inFIG. 9. It can be shown mathematically, however, that an elementstructure which influences the amplitude of the waves, lets through afar smaller portion of the wave energy than an element structure whichinfluences the phase of the waves. It can be mentioned here that thetheoretical maximum efficiency of an ideal amplitude grid (i.e.consisting of very rigid or stiff elements) is 12.5%, while an idealphase grid has a theoretical efficiency of 100% as a maximum. It willoften be of interest to have an element structure which is a combinationof an amplitude grid and a phase grid, since a single installationapparently may include stopping elements influencing amplitude, as wellas delaying elements influencing phase.

Although it is still too early to show which amounts of energy can betaken from sea waves by means of arrangements according to theinvention, a moderate estimate shall here be given. A reasonable figurefor the average power of the sea waves in the North Atlantic is 50 kWper meter. An arrangement for concentrating sea wave energy can forexample extend over ten kilometers. Nominally then 500 MW is available.With a modest assumption it is believed that this power can be broughtto the focal area of a phase grid with an efficiency of 50%, and thatthe energy tapping in the power station can take place at an efficiencyof 40%. These assumptions give a power station producing an average of100 MW. The production per year will be 800 GWh, which is comparable tothe development of a major watercourse.

An element structure as indicated in FIG. 3 was dimensioned for tests ina large water tank. The results of the tests are tentatively illustratedin FIG. 4. In FIG. 4 two of the walls of the tank are designated 41. Thewater surface is designated 42. In the tests there was employed a gridformed of a limited number of openings or passages, namely twelveopenings (of which only eight openings 44a-46h are shown in FIG. 4)forming one half of an amplitude grid which was divided by means of awall 44 which was painted white in order to make it easier to study thewave picture adjacent to and at the focus 45. The openings 46 in thegrid structure were defined by stopping elements 43 mounted in such away that they reached the bottom of the tank. The depth was 0.5 m.Behind the elements 43 there was provided a wave generator (not shown)which emitted plane waves with a wave height of about 2 cm against thegrid.

In these tests there was employed a wave length of 25 cm. The focaldistance, i.e. the distance from the arrow 45 was 220 cm. The length ofthe grid was about 450 cm. The width of the stopping elements and of theopenings appear from the table below, in which A is the distance betweencorner 47 and the nearest edge of an opening and B is the distancebetween the corner 47 and the remote ends of the corresponding opening,as seen in the measured direction.

    ______________________________________                                        Opening         A       B                                                     number          cm      cm                                                    ______________________________________                                        1               10.0    75.9                                                  2               108.3   134.3                                                 3               156.9   177.6                                                 4               196.9   215.2                                                 5               232.7   249.6                                                 6               266.1   282.1                                                 7               297.8   313.2                                                 8               328.3   343.3                                                 9               358.0   372.6                                                 10              387.1   401.4                                                 11              415.6   429.7                                                 12              443.7   457.6                                                 ______________________________________                                    

With interference the waves in these experiments are propagated mainlyas circular waves in the direction towards focus, and against the wall44 it can be seen how the wave height builds up and reaches its maximumin at the focus (marked by arrown 45), and the wave height thereafterdecreases. The wave height immediately after the openings 46 was about 1cm and the wave height in focus 45 was about 4 cm.

It is remarked that these laboratory experiments were conductedexclusively to show the principle and that these are not meant as ameasure of what can be obtained with sea waves in practice.

In FIG. 5 there is shown a structure 51 carrying 23 openings and 24stopping elements 51a located outside a part of a coast. The elements51a are arranged along a straight line and transversally to the dominantwave incident direction as indicated with arrow 52. The elements 51a areso positioned and mutually arranged that the focal area 53 is located ina bay outside a naturally existing or blast-out rock cleft which leadsto an elevated basin as indicated at 54. A conventional power station 55with a high voltage transmission network is built on the lee side.Alternatively, the wave energy in the focal area can drive a largehydraulic pump for lifting water into the basin 54.

In FIG. 6 there is shown a structure 61 of delaying elements 61a, 61band 61d located with an extension of some kilometers at sea. In thefocal area there is located a platform 62 having a scoop wheel 63 orother means for utilizing the concentrated wave energy. As analternative the platform 62 may comprise a pump arrangement, a basin anda turbine house. Electric power being produced can be conducted to theshore through a sea cable. The elements 61a and 61d consist of anchoredair-filled hoses, whereas the elements 61b consist of anchored buoys.Some of the elements can be anchored to remotely controlled winches 64.

In FIG. 7 there is shown an amplitude grid consisting mainly of stoppingelements in the form of islets, rocks and skerries 71, which do notnecessarily have to lie on a straight line. Other elements 72 are formedby blasting of an islet. The grid is completed by building stoppingelements 73. Alternatively, these could be delaying elements. A platform74 with equipment for transforming wave energy to mechanical/electricalenergy is located in the focal area of the grid. A steep coast part 75with deep water serves to reflect waves back towards the focal area inwhich there will exist a standing wave pattern. There is also provided aphase grid 76 preferably at a short distance from the steep part 75 forcontributing to the concentration of the reflected waves at the focalarea. In the calculation in this case it must be taken into account thatthe grid 71, 72, 73 interacts with the reflecting part 75 and with thephase grid 76 and possibly also that there is provided for tapping ofenergy from the wave field.

In FIG. 8 there is shown a coral-reef 81 modified as an amplitude gridby blastings 82 for concentrating sea wave energy at an offshore powerstation 83 in the short-range field of the grid. In the Figure there areshown 13 openings, while 20 to 50 would be natural. Since there arecoral-reefs which can form an aperture of as much as 1000 kilometers, afew hundred openings will be of interest is such cases. In trade windregions the wave length is about 300 meters. In the focal area the waveheight can be expected to be 50 meters, which indicates the large amountof energy available.

In FIG. 9 there is shown an anchored delaying element 91 formed as acylinder or plate having horizontal dimensions of an order of magnitudecorresponding to 1 wave length, and being anchored to the bottom,whereas the element 92 is a corresponding object, but curved in one oranother direction depending upon the conditions. The elements 91 and 92may be connected to each other as indicated with a dotted line. Becauseof the size of the elements it can be practical to build them up of moreeasily handled sections. The element 93 is a cubical air tank whereasthe element 94 represents a large ship which may be anchored or kept inposition by its own machinery. The element 95 is a natural shelf.

The generally plane and plate-like element 91 shown in FIG. 9 will, in asubstantially horizontal orientation in the water, essentially haveinfluence on the phase of the waves. Such an element should in this casebe located so deep in the water that the deepest troughs of the waves tobe influenced, do not reach down to the element or elements, i.e., thewaves should normally not break thereover.

In FIG. 10 there is shown a stopping element 101 formed as a concreteblock being anchored to the bottom by means of legs. Moreover, there isshown a cylindrical element 102 with an anchoring rope fixed to thebottom. The element 103 is a wrecked ship whereas the element 104 is anartificial rock.

In FIG. 11a there is indicated an amplitude or phase grid 111 forfocusing plane waves 112 as shown with concentric rings 113. In front ofthe focal area in the short-range field there is located another grid114 for example a phase grid which delays the phase of the circularwaves outwardly to both sides from the center of the grid 114 so thatplane waves 115 with enlarged wave height propagate in the direction ofthe arrow. Alternatively, the grid 114 can be formed of a natural bottomformation as indicated in FIG. 11b. Here the bottom portion 117 forms aphase grid. Thus, FIG. 11b shows a transverse section in a planecorresponding to grid 114 with a symmetric bottom contour 117 on bothsides of a center line or axis CL. The shallower portions withincreasing distance from line CL result in increasing delays of the wavefront, so that the incident converging circular wave 113 is transformedinto a plane wave 115 of reduced width. Accordingly, grid 114 or bottomportion 117 have a structure and an effect being the inverse of grid111.

The set-up of FIG. 11a can be used for energy production as well as forthe breaking of ice. Thus, grid 114 may have provisions for angulardisplacement in azimuth so that the plane waves 115 can be directed atdifferent angles into the ice for breaking a correspondingly fan shapedlane therein. The angular displacement of grid 114 can be brought aboutby individually moving elements of the grid, such elements being forexample of the same type as elements 61a in FIG. 6.

With the focal area of the grid 111 located near to or at the shore aconstructed channel 116 can lead the waves 115 to an elevated basin forpower production.

The grids 111 and 114 in the arrangement of FIG. 11a can also beregarded as parts of a single larger element structure, in which thegrid 111 forms a concentrating main structure part and the grid 114forms a dispersing auxiliary structure part located in the short-rangefield from the grid 111.

The specific example of a structure of delaying elements shown in FIG.12 is calculated for a wave-length of 150 m, and the drawing is scaleddown so that one cm represents 60 m of actual size. FIG. 12 shows onlyone half of the complete structure (line A--A' being an axis ofsymmetry). There is shown the contour of five elementsAA'B-BB'C-CC'D-DD'E and EE'F. These elements are designed to be locatedhorizontally in the water at a depth of 16.8 m. The thickness of theelements is determined by constructional and strength considerations,and may be for example 5 m.

The structure is designed to have an acceptable concentration effectwithin a variation of 10% in the wave period.

The arrow in FIG. 12 shows the direction of wave incidence.

I claim: .[.1. A system for concentrating water wave energy, said systemcomprising an off-shore grid-like means for transforming substantiallyrectilinear fronts of incident water waves into substantially arch-likewater wave fronts with concentrated wave energy in a common central areaspaced from said grid-like means, said grid-like means comprising astructure formed of a series of interacting water wave stopping and/ordelaying elements arranged substantially transversely of a primarydirection of propagation of said incident water waves and said elementsbeing relatively dimensioned such that elements and spacingstherebetween in a central portion of said series of elements have largerdimensions in the longitudinal direction of said series than elementsand spacings therebetween remote from said central portion..]. .[.2. Asystem as claimed in claim 1, wherein said common central area issituated behind said grid-like means, as viewed in said primarydirection of propagation of said incident water waves..]. .[.3. A systemas claimed in claim 2, wherein said concentrated wave energy is to beutilized for power production, and wherein said common central area islocated within a sector defined by: ##EQU2## wherein: D_(max) =thelargest dimension of said series of stopping and/or delaying elements;and elements..]. .[.4. A system as claimed in claim 2, wherein saidgrid-like means further comprises wave-diverging means positionedbetween said series of elements and said common central area..]. .[.5. Asystem as claimed in claim 2, further comprising wave reflection meanspositioned behind said grid-like means and behind said common centralarea, as viewed in said primary direction of propagation of saidincident water waves..]. .[.6. A system as claimed in claim 1, whereinthe width of said grid-like means, as measured along said incident waterwaves, exceeds the largest wave lengths of said waves..]. .[.7. A systemas claimed in claim 1, wherein said grid-like means comprises at leastone said element in the form of an interacting natural formation, forinstance a rock, that may have been artificially shaped to improveinteraction with neighboring said elements, for instance by blasting..]..[.8. A system as claimed in claim 1, wherein at least one of saidelements is located below the water surface..]. .[.9. A system asclaimed in claim 8, wherein at least one of said elements is resilientto said incident water waves..]. .[.10. A system as claimed in claim 8,wherein at least one of said elements comprises a plate-shaped memberwhich is substantially horizontally oriented in the water at a depththerein that said incident water waves do not break on the member..]..[.11. A system as claimed in claim 1, wherein at least one of saidelements is adjustable in size and shape in a controlled manner..]..[.12. A system as claimed in claim 1, further comprising means,associated with at least one of said elements, for displacing said atleast one element with respect to position and/or orientation in acontrolled manner..]. .Iadd.13. A lens system for concentrating waterwave energy, said system comprising an off-shore grid-like means fortransforming substantially rectilinear incident water wave fronts intocurved water wave fronts which converge toward a common focal areaspaced beyond said grid-like means, said grid-like means comprising astructure extending in a longitudinal direction generally transverselyof a primary direction of propagation of said incident water wavefronts, said structure comprising means for changing the phase of saidincident water wave fronts by effecting a delaying action on a waterwave front passing over said structure, said phase changing meanscomprising a plurality of delaying elements aligned in said longitudinaldirection and completely submerged at a depth such that the water wavesdo not break on passing over said elements, each said delaying elementbeing shaped and positioned in said longitudinal direction such thatsaid delaying action on a water wave front passing over said element isgreater above locations of each said element nearer a central portion ofsaid structure than above locations thereof more remote therefrom, saidplurality of delaying elements being shaped and mutually positioned insaid longitudinal direction such that said delaying elements forminteracting means for creating a greater delaying action nearer saidcentral portion of said structure than remote therefrom, whereby saidcurved water wave fronts are formed and roll beyond said structuretoward said focal area..Iaddend. .Iadd.14. A system as claimed in claim13, further comprising wave diverging means positioned between saidplurality of delaying elements and said focal area. .Iaddend. .Iadd.15.A system as claimed in claim 13, further comprising water wavereflection means positioned behind said structure and behind said focalarea, as viewed in said primary direction of propagation of saidincident water wave fronts. .Iaddend. .Iadd.16. A system as claimed inclaim 13, wherein at least one said delaying element comprises aninteracting natural formation, for instance a rock, that may have beenartificially shaped to improve interaction with neighboring saiddelaying elements..Iaddend. .Iadd.17. A system as claimed in claim 13,wherein at least one of said delaying elements is resilient to saidwater waves. .Iaddend. .Iadd.18. A system as claimed in claim 13,wherein at least one of said delaying elements comprising a plate-shapedmember extending generally horizontally in the water, portions of saidmember toward said central portion of said structure extending upwardlyin the water to a greater height than portions of said member moreremote from said central portion of said structure. .Iaddend. .Iadd.19.A system as claimed in claim 13, wherein the size of at least one ofsaid delaying elements is adjustable. .Iaddend. .Iadd.20. A system asclaimed in claim 13, wherein the shape of at least one of said delayingelements is adjustable..Iaddend.